A few possible scenarios

[iaaems]

Severe Weather: This can happen anytime - usually with some advance notice.
Severe Economic Downturn: We might just be in the middle of one of those right now.
Climate Change: I am told that the Jet Stream moves now and then which probably explains our longer than usual winter - but isn't the 'climate' always changing?
Pandemic: One or two of those over the last fifty years or so in different parts of the world including this country.
Of all the items on the list given I would think that the above are more likely to happen.
Nuclear Incident, Total Economic Collapse, M.A.D. Global Nuclear War: There are so many vested interests about these days that the likelihood of any of these happening are probably less likely than the first four - AT LEAST I WOULD HOPE SO.
As I am a total newbie to 'the cause' you might think that I am being a little bit simplistic - but my comments above are based on my life experiences so far.